Batteries are commonly used as electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized. The cathode contains an active material that can be reduced. The anode active material is capable of reducing the cathode active material.
When a battery is used as an electrical energy source in a device, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
Commercial primary alkaline batteries often include cathodes including manganese dioxide and anodes including zinc. Such batteries often have a cylindrical housing and come in standard AA, AAA, AAAA, C, and D sizes defined by International Electrotechnical Commission standards. Each manufactured alkaline battery contains an amount of cathode and anode active materials that is limited by the internal volume of the battery, and has a theoretical amount of electronic discharge capacity determined by the amount of the active materials. For example, an AA, AAA, AAAA, C, and D battery has a theoretical discharge capacity of about 3.7 Ah (Ampere hour), about 1.5 Ah, about 0.7 Ah, about 10.4 Ah, and about 22.9 Ah, respectively.
In use, the total amount of capacity that can be extracted from the battery during the lifetime of the battery is less than 100% of the theoretical capacity of the battery. This can be due to three kinds of losses: Ohmic losses, activation polarization losses, and concentration polarization losses of the capacity. In particular, concentration polarization losses of the battery capacity occur when the active materials are depleted or reaction products build up excessively within a local region of the battery, for example, near one or both of the electrodes. The depletion of the active materials occurs when the rate of consumption of the active materials exceeds the rate of active material replacement by means of diffusion, electro-migration, osmosis, or other mechanisms. A build-up of reaction products occurs when the rate of generation of reaction products exceeds the rate at which the generated reaction products escape from the reaction zone by diffusion, electro-migration, osmosis, or other mechanisms. When either of the active material depletion or the reaction product build-up takes place, an unfavorable shift in operating voltage for the electrodes of the battery can occur. This results in a lower battery operating voltage and the voltage of the battery can fall below the operating voltage of a device before the battery is fully discharged.
Ohmic losses can be due to either electronic or ionic resistances in the battery. In particular, ionic losses from ionic resistances can occur based on the physical state of the electrolyte, for example, the electrolyte can be free or can be absorbed in a matrix of solids, such as the pores of the anode, cathode, or separator. The ionic resistance is the lowest when a maximum ionic conductivity of the electrolyte is reached when the electrolyte is substantially in the form of pure liquid electrolyte. When the electrolyte coexists with solid phase materials, the ionic resistance can increase based on the limitation of the ionic conduction paths in the electrolyte to channels in the solid matrix that are filled with electrolyte. These channels follow paths through the inter-connected pores and can be long and tortuous. The cross sectional area available for ionic conduction in the electrolyte phase can also be limited by the small pore diameters along the paths. A high ionic resistance can give rise to significant Ohmic losses as the battery is discharged. The ionic resistances can exist in a fresh, as-manufactured battery, due to the existing pore structures of the anode, cathode, and separator and can change during the use of the battery